1. Field of the Invention
The invention relates, in general, to respirators and, in particular, to a new and useful respirator comprising a respiratory air circulation unit with an adjustable displacement volume and a respiratory air supply line and a respiratory system comprising an inspiration loop, an expiration loop and valve elements mounted in these loops and controllable by means of a control unit
Such a respirator can be operated with a half-open or semi-open respiratory system without counter respiration or with a partially or completely closed respiratory system having counter respiration.
In known respirators with the option of counter respiration part of the expiration gas or all of the expiration gas is led back into the respiratory system and it is re-supplied to the patient during the following inspiration strokes of the respiratory air circulation unit once it is depleted of the exhaled CO.sub.2 and enriched with a fresh gas and, in case of narcosis, with the required anesthetic. A half-open respiratory system is e.g. achieved when the fresh air supply flow is adjusted larger than or equal to the respiratory air volume per minute In this case the patient receives fresh respiratory air only and the used respiratory air is discharged through the discharge valve. The respiratory air circulation unit of the respirator takes in the respiratory air from the expiration loop and empties it into the inspiration loop. For this purpose the circulation unit is usually equipped with an adjustable volume, e.g. in the form of a bellows or a piston movable in a cylinder. Such a respirator is described in German Patent No. 34 34 908.
The amount Of the displacement volume required for the inspiration phase is determined by the displacement volume of the bellows or the piston-cylinder unit. Typical displacement volumes in adults are 0.5 to 1.5 liters, in extreme cases up to 2 liters, in premature infant and newborns, however, they are merely some 10 milliliters. A respirator whose circulation unit is calibrated for the large displacement volumes of adults can be used for a volume-constant artificial respiration of infants, premature infants or newborns only to a limited extent due to the inferior dosing accuracy for small displacement volumes. However, because of the small lungs of infants volume stability is indispensable.
Furthermore, a large displacement volume of up to 2 liters results in such a high unit-compliance that a volume-stable dosing of the small displacement volumes is jeopardized. The major space requirement of the known respirators, which have unnecessarily large displacement volumes when used for the artificial respiration of infants are regarded as negative. The option of equipping the respirator with special devices for the respiration with small displacement volumes, e.g. respectively smaller bellows or piston-cylinder units, does not result in improvements, as the respective scaling has to be changed also. Furthermore, this process incurs additional costs as piston-cylinder units or bellows of various sizes have to be stored.